A known hybrid drive system is used both for road vehicles, for example passenger cars and buses, and for rail vehicles. It is distinguished by the fact that an internal combustion engine with a generator, for example a diesel engine with a generator, and an electric motor are present. The internal combustion engine is connected to the electric motor via the generator and an intermediate circuit. The intermediate circuit is assigned a battery or a capacitor as the energy accumulator. In comparison with a battery, a capacitor has a higher power density, a longer service life and is also maintenance-free. To be able to store an adequate amount of energy, special capacitors, known as ultracapacitors, are used.
In the case of the vehicle, a distinction is made between stationary mode, starting mode, driving mode and braking mode. In stationary mode and during starting, energy is taken from the energy accumulator. In the case of urban buses, starting advantageously takes place exclusively electrically, in order that the energy stored in the energy accumulator is efficiently used and no exhaust gases are emitted, for example at a bus stop. During driving mode, the energy accumulator is neither discharged further nor charged. Only during braking mode is the kinetic energy of the vehicle to be stored in the energy accumulator.
Owing to a relatively low energy density, in the case of a capacitor the voltage drops relatively quickly when discharging occurs. If the capacitor is operated directly on the intermediate circuit of the drive system, the voltage of the intermediate circuit, which is then also low, has a disadvantageous effect on the performance of the drive system, i.e. the drive components must be operated with low voltage.
It is known to counteract this disadvantage of low voltage by generating a high current flow, which is possible only by way of overdimensioned components. It is also already known to connect the capacitor to the intermediate circuit via a converter, for example via a step-up/step-down converter. However, this converter includes complex and expensive electronics and also causes an appreciable additional weight.
Both the overdimensioning of the components and the converter lead to a greater weight and higher costs.